1. Field of the Invention
The present invention relates to an apparatus for reading an original such as characters, figures or the like and converting it into corresponding electrical signals. In particular, the invention concerns a photoelectric converter apparatus suited for use in a pattern reader system in which an original such as a slip, bill or the like carrying thereon pattern information such as characters, figures or the like is irradiated with light from a light source, and the reflected light from the irradiated original is converted into electric signals representing the pattern information.
2. Description of the Prior Art
In systems such as facsimiles and OCR's (optical character readers) conversion of pattern information such as characters, figures or the like into corresponding electrical signals is performed. Photoelectric conversion elements of a charge storage type and adapted to constitute the photoelectric converter apparatus for the pattern reader for converting the patterns of characters, figures or the like into corresponding electric signals. Typically, the charge storage type photoelectrical conversion element consists of a photodiode in which electric charge is stored under irradiation with light.
Such pattern reader system generally include a light source for projecting light to an original such as slip, bill or the like carrying thereon alphanumeric characters, figures or the like, a pattern reader apparatus comprising a photoelectric conversion apparatus having a plurality of charge storage type photodiodes arrayed in a linear row, and a reading circuit for reading out sequentially electric charges stored in the individual photodiodes.
Reflected light from the original which is irradiated with light from the light source, is collected through a collector lens and applied to the photodiodes of the photoelectric converter apparatus. After a predetermined time duration has elapsed, the stored charges are outputted to the reading circuit. This process is repeated many times while the original being progressively moved.
Since the quantity of light reflected from blank or white portions of the original is large, the quantity of electric charge stored in the photodiodes which are positioned opposite to the white portions will be correspondingly large. On the contrary, the quantity of stored charge in the photodiodes facing toward the black portions of the original will be small. In this manner, electrical signal representing the lights and shades of the original can be obtained. An electrical signal of an analog nature thus obtained may be processed as it is. However, it is common to convert it into digital signal of logic "1" and "0" through comparison with a threshold reference.
A fluorescent lamp is well suited to be used as the light source for the pattern reader system in view of a high light emission efficiency, an advantageous geometrical configuration for illuminating the original over the whole width thereof, and the possibility of assuring a relatively uniform sensitivity for the individual photodiodes constituting the photoelectric converter apparatus. On the other hand, the fluorescent lamp suffers from such drawbacks that the luminance thereof is likely to undergo influences of variations in the source voltage, the ambient temperature and the illuminating duration. For example, the luminance of the fluorescent lamp will vary remarkably for several minutes after the initial firing. In reality, the ratio of the maximum luminance to the minimum luminance over the use life will amount to about 3 or 4. Such variation in luminance will give rise to an excursion in the output signal level of the photoelectric converter apparatus as a function of time and eventually involve erroneous pattern reading.
Heretofore, several attempts have been made to prevent the variation in the luminance of fluorescent lamps with a view to attaining a stabilized output from the photoelectric converter apparatus. One known method resides in the provision of a light intensity regulator system according to which the luminance of the light source of fluorescent lamp is detected to produce a control signal for controlling the voltage of a power supply source for the lamp through a feedback loop so that the luminance of the lamp may be maintained essentially constant. According to another known method, the charge signal from the photoelectric converter apparatus is amplified in the reading circuit with the gain varied in conformance with the variation in the luminance of the light source, thereby to attain a constant output level of the pattern reader apparatus. However, since the fluorescent lamp is generally energized from an a.c. source such as a commercial power line, the luminance of the lamp can not avoid a pulsating variation synchronized with the cycle of the alternating voltage. Under such circumstances, the circuitry required for carrying out the known control methods described above has to be operated in synchronism with the pulsation of source voltage, which of course involves much a complicated and expensive circuit configuration.
As another attempt, it is also known to change over the charge storing duration of the photodiodes. More specifically, a plurality of different charge storing durations corresponding to n.T where n represents an integer and T represents a period of a.c. source voltage, are previously made to be available for the photodiodes and selectively changed over in dependence on the luminance of fluorescent lamp. For example, two kinds of charge storing durations twice and three times as long as a period of the a.c. source voltage are made available for the photodiodes of the photoelectric converter apparatus. When the luminance of light source is high, the photodiodes are allowed to store electric charge for duration twice as long as a period of the a.c. source voltage. On the other hand, when the luminance of lamp is low, i.e. the photodiodes store electric charge at a relatively low rate, the longer storage duration of three times as long as a period of a.c. voltage is used. After the selected duration has elapsed, the stored charge is transferred to the reading circuit. In this manner, protection is obtained against the occurrence of remarkable difference in the output level of a the reader circuit.
The reason why the charge storing duration must have lengths equal to a period of a.c. source voltage multiplied by integers can be explained by the fact that the storage of charge can then be initiated at any phase position of the pulsating cycle of the luminance without incurring inconsistency in the quantity of stored charge in the photodiodes.
The above known method is, however, disadvantageous in that the output level of the reading circuit will undergo an abrupt change immediately after the charge storing duration has been changed over, which will eventually result in an erroneous pattern reading.